Mumbai has 3 UNESCO World Heritage Sites namely CSMT Station Building, The Victorian and Art Deco Ensemble of Mumbai Fort Area (Mumbai High Court and University etc. which are near CSMT) and Elephanta Caves (ferries available from Gateway of India). Nariman Point, Marine Drive, Hazi Ali, Chowpati Beach, Juhu Beach etc are other places which may be covered. Best will be to take the Mumbai Darshan Bus and see all these places in one day (except Elephanta Caves for which you need to plan separately). Shri Siddhi Vinayak Temple is also must visit. If interested then you may plan for Kanheri Caves - Sanjay Gandhi National Park (near Borivali) also.
You can divide your time on the basis of what you want to see
example : 1) Culture and Heritage - Elephanta caves
Gateway of India
Taj hotel
2) Architecture - In South Mumbai, you will find different styles of architecture.
3) Museums/ Gallery - Jehangir Art Gallery
Prince of Wales museum
4) Temple - Siddhivinayak temple
Mahalaxmi temple
Babulnath temple
Iskon temple
Haji Ali Dargah
Mt. Mary Church
5) Beach - Juhu Chowpaty
Girgaon Chowpaty
6) Marine lines ( Don't forget to have Rustom's icecream)
I will share some offbeat places which might be useful to you when u plan travel to mumbai. Sassoon dock
Pandavkada falls
Dahanu beach
Gilbert hill
Chhota Kashmir
Sewri fort
Mahakali caves
Jawhar
Sandhan valley
are some spots to explore in mumbai.
https://tripoto.page.link/dgQBsfzmA3gpXbVNA
Kindly check the above link for more details on spots to visit in mumbai.